Petroleum refiners often produce desirable products such as naphtha and gasoline as well as higher boiling liquids, such as turbine fuel, diesel fuel, and other hydrocarbon liquids known as middle distillates, by hydrocracking a hydrocarbon feedstock derived from crude oil. Hydrocracking also has other beneficial results such as removing sulfur and nitrogen from the feedstock by hydrotreating. Feedstocks most often subject to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation.
Hydrocracking is generally carried out by contacting, in an appropriate reactor vessel, the gas oil or other hydrocarbon feedstock with a suitable hydrocracking catalyst under appropriate conditions, including an elevated temperature and an elevated pressure and the presence of hydrogen so as to yield a lower overall average boiling point product containing a distribution of hydrocarbon products desired by the refiner. Although the operating conditions within a hydrocracking reactor have some influence on the yield of the products, the hydrocracking catalyst is a prime factor in determining such yields.
Hydrocracking catalysts are subject to initial classification on the basis of the nature of the predominant cracking component of the catalyst. This classification divides hydrocracking catalysts into those based upon an amorphous cracking component such as silica-alumina and those based upon a zeolitic cracking component such as beta or Y zeolite. Hydrocracking catalysts are also subject to classification on the basis of their intended predominant product of which the two main products are distillate and naphtha, a term which in the hydrocracking refining art refers to distillable petroleum derived fractions having a boiling point range that is below that of distillate. Naphtha typically boils in the range of from the boiling point of hydrocarbons having six carbon atoms per molecule (i.e., C6) to 216° C. (420° F.) and includes the product recovered at a refinery as gasoline. At the present time, naphtha and gasoline are in high demand. For this reason, refiners have been focusing on hydrocracking catalysts that selectively produce a naphtha fraction.
The three main catalytic properties by which the performance of a hydrocracking catalyst for producing naphtha is evaluated are activity, selectivity, and stability. Activity may be determined by comparing the temperature at which various catalysts must be utilized under otherwise constant hydrocracking conditions with the same feedstock so as to produce a given percentage, normally about 65 percent, of products boiling below 216° C. (420° F.). The lower the temperature required for a given catalyst, the more active such a catalyst is in relation to a catalyst requiring a higher temperature. Selectivity of hydrocracking catalysts for producing naphtha may be determined during the foregoing described activity test and is measured as a percentage of the fraction of the product boiling in the desired naphtha product range, e.g., C6 to 216° C. (420° F.). Stability is a measure of how well a catalyst maintains its activity over an extended time period when treating a given hydrocarbon feedstock under the conditions of the activity test. Stability is generally measured in terms of the change in temperature required per day to maintain a 65 percent or other given conversion.
Although cracking catalysts for producing naphtha are known and used in commercial environments, there is always a demand for new hydrocracking catalysts with superior overall activity, selectivity, and stability for producing naphtha.